Investigation of the voids formation process in friction stir welding by numerical modeling and experiment on the basis of a stress-condition-hypothesis

搅拌摩擦焊（FSW）（包括电辅助搅拌摩擦焊- EAFSW）的实际应用表明，可能会形成内部缺陷，如空洞。它们对焊接接头的力学性能有显著的影响。这些空洞形成的原因尚不清楚，需要对FSW中形成塑性区的特点进行详细的研究，并分析相应的应力条件。基于实验和物理数学模型对塑性区现象进行的最新研究尚未对空洞形成过程的原因提出任何解释和标准。其中一个主要的影响因素是，在FSW条件下，塑化金属在塑性区内的变形特性是未知的。基于标准实验程序（热压缩、扭转、霍普金森杆试验等）的不同本构材料定律不允许测定（例如，应变率、动态粘度）。该项目的目标是在详细调查塑性区内应力和应力条件假设的基础上，制定FSW （EAFSW）中塑性区空洞形成过程起源的一般标准。为了实现这一目标，将开发一个FSW （EAFSW）过程的复杂热力学模型，描述热和力学现象，即热产生，热传播，塑性变形，粘性流动。该模型将考虑FSW的实际操作参数（EAFSW），金属在FSW条件下的温度依赖热物理和变形特性，并产生由焊接工具引起的塑性应变，以及工件中的热量产生，传热和传质。模型所需的材料（工具、工件、背板）的温度相关热物理特性将从本地isf数据库和文献中获取。通过对接收到的实验数据进行计算处理，可以得到塑性区金属的力学性能。经过校准和验证，FSW （EAFSW）的耦合热-力学模型将允许计算材料粘性流动的主要特征：流动速度的分布，应变率，金属在塑性区内压力和应力张量的分布。在提出应力条件假设的基础上，对得到的内压和应力分布进行详细分析，可以提出孔洞形成的一般判据。

The practical usage of friction stir welding (FSW) (including electrically assisted friction stir welding - EAFSW) showed that internal defects such as voids may form. They have a significant influence on the mechanical properties of the welded joints.The reason of the formation of these voids is unclear and requires a detailed investigation into peculiarities of building the plastic zone in FSW and an analysis of the corresponding stressed condition. Conducted up-to-date studies, based on experiment and physical-mathematical modeling of the phenomena in the plastic zone, have not presented any explanations and criteria on the reasons of the voids formation process. One of the main factors here is that the deformation properties of the plasticized metal in the plastic zone under the conditions of FSW are unknown. Different constitutive material laws, based on the standard experimental procedures (hot compression, torsion, Hopkinson bar tests, etc.), do not allow their determination (for example, strain rate, dynamic viscosity).The goal of the project is to formulate general criteria for the origin of the voids formation process in the plastic zone during FSW (EAFSW) on the basis of both a detailed investigation into internal stresses in the plastic zone and a stress-condition-hypothesis.To reach this goal a complex thermo-mechanical model of the FSW (EAFSW) process, describing thermal and mechanical phenomena, namely heat generation, heat propagation, plastic deformation, viscous flow will be developed.The model will consider the real operational parameters of FSW (EAFSW), the temperature dependent thermo-physical and deformation properties of the metal in the conditions of FSW and produce plastic strain induced by the welding tool, as well as heat generation, heat and mass transfer in the workpiece.The temperature dependent thermo-physical properties of the materials (tool, workpiece, backing plate), needed for the model, will be taken from a local ISF-database and literature. The mechanical properties of the metal in the plastic zone will be obtained during FSW-alike trials with processing the received experimental data by a calculation procedure.After calibration and verification, the coupled thermo-mechanical model of FSW (EAFSW) will allow the calculation of the main characteristics of the material viscous flow: distribution of the flow velocities, strain rate, distribution of the internal pressure and stress tensor of the metal in the plastic zone. Based on the proposed stress-condition hypothesis and analyzing the obtained distributions of the internal pressure and stresses in details, it will be possible to propose general criteria for the voids origination.